Use torque wrench when checking head bolts for tightness.

Performance Gain:

Repair broken head bolts

Complementary Modification:

Replace valve cover gasket

Overheating your BMW E46 can lead to a major engine failure. Sustained high engine temperatures can weaken the structure of the aluminum block assembly in the area of the bolt threads. Usually this issue is found out when reassembling the cylinder during a head gasket replacement. When tightening the cylinder head bolts, the threads strip out of the block and the bolt cannot be tightened to the correct torque specification. Once the engine has been overheated to the point of the head bolt threads stripping, it is likely beyond repair. Cylinder head and block warpage is likely, along with all plastic engine attachments. The engine block is not repairable and BMW suggests replacing with a new short block. If your engine has overheated and the cylinder head gasket has failed, inspect the head bolt threads (the ones in the block) before beginning a major repair.

In this tech article I will over how to inspect the cylinder head bolt threads in the block for stripping.

Using a small flat-head screwdriver; release ignition coil electrical connector retainer by prying up, then pull electrical connector out of ignition coil (green arrow). Do this for all six ignition coils.

Remove each ignition coil from cylinder head by pulling straight up. If coil resists, twist when pulling up to break free from spark plug. The ignition coil rubber boot can become stuck to spark plug over time. Remove all six coils.

Once valve cover has been removed, check head bolts. Do this by attempting to torque them to factory spec. The bolts located at the rear of cylinder head usually fail first. This is a good place to start. I like to start with the bolts between cylinders 5 and 6 (green arrows). If you do not find any faulty head bolt threads, you are OK to replace the head gasket. Remember to check the cylinder head and block for warpage when you remove them to ensure proper sealing and to avoid future problems. Use a straight edge on the block surface, as well as the cylinder head, to determine if any deviation caused by overheating is present. Also inspect for any protrusion of the cylinder liners above the surface of the engine block.

Comments: Unfortunately, I had already removed the head when I came across this article. I know I'll need new head bolts for the final installation but, is it ok to put the head back on with the old gasket and try re-torqueing with the original bolts just to check that the threads in the block aren't stripped? I'd hate to have to buy an additional new set of bolts just so I can perform this check.

August 13, 2016

Followup from the Pelican Staff: If you plane to do that, use an old gasket. The bolts will work fine for testing, but not for reinstalling the head permanently. - Nick at Pelican Parts

covertaxn

Comments: If the torque spec calls for 40nM + 90 and + another 90, repeating this on tight head bolts will strip the threads out of the block would it not?

July 21, 2016

Followup from the Pelican Staff: You're not performing the entire torque sequence. This is the method used by professionals to check for stripped bolts. Once you have access to the bolt, apply the suggested torque (from BMW), the bolts should not turn. Your torque wrench will click off, as the max torque is already reached. - Nick at Pelican Parts

David

Comments: so by checking that the block can hold 40nM, that's enough to expect it to hold 180 degrees of torque to yield? i've seen a block give up the threads on the second 90 degrees. and proper torque should be checked from loose. a better check might be to see how much it takes to break the static torque while trying to tighten a couple extra degrees of rotation. but in that case, a beam-type torque wrench is needed and perhaps an observer. or loosen the bolts and start each one from scratch before disassembly. is there a better method?

May 5, 2015

Followup from the Pelican Staff: I check it the way described, I don't loosen the bolts as to not break the bond of the gasket and cylinder head.

I am open to your method, however only have experience with mine. - Nick at Pelican Parts

robhaugen

Comments: Excellent guide but should i loosen the head bolts and then re-torque them?
What are the torque specs?
Thanks